fix(highlight): split old/new treesitter parsing

Problem: highlight_treesitter concatenated all hunk lines (context, -,
+) into a single string. Mixed old/new code produced invalid syntax
(e.g. two return statements), causing treesitter error recovery to drop
captures on lines after the syntax error.

Solution: split hunk lines into two versions — new (context + added)
and old (context + deleted) — each parsed independently. Use a line_map
to resolve treesitter row indices to buffer lines, with the old version
only mapping deleted lines to avoid duplicate extmarks on context.

Also fixes three related issues exposed by the improved TS coverage:

- Replace Normal extmark with DiffsClear (explicit fg from Normal.fg).
  Normal in extmarks doesn't reliably override vim :syntax foreground.

- Reorder priority stack to DiffsClear(198) < syntax(199) < line
  bg(200) < char bg(201). TS capture groups can carry colorscheme
  backgrounds that would override diff line backgrounds at higher
  priority.

- Gate DiffsClear on per-line coverage tracking. Only clear fugitive
  syntax fg on lines where TS/vim actually produced captures, preventing
  force-clearing on lines where error recovery drops captures.

lua/diffs/init.lua

@@ -186,6 +186,7 @@ local function compute_highlight_groups()
   local blended_add_text = blend_color(add_fg, bg, 0.7)
   local blended_del_text = blend_color(del_fg, bg, 0.7)
 
+  vim.api.nvim_set_hl(0, 'DiffsClear', { default = true, fg = normal.fg or 0xc0c0c0 })
   vim.api.nvim_set_hl(0, 'DiffsAdd', { default = true, bg = blended_add })
   vim.api.nvim_set_hl(0, 'DiffsDelete', { default = true, bg = blended_del })
   vim.api.nvim_set_hl(0, 'DiffsAddNr', { default = true, fg = add_fg, bg = blended_add })